fix: filter empty text/reasoning parts before sending to LLM (#23284)

## Problem

Anthropic rejects requests containing empty text content blocks with:

```
messages: text content blocks must be non-empty
```

Empty text parts (`""` or whitespace-only like `" "`) get persisted in
the database when a stream sends `TextStart`/`TextEnd` with no
`TextDelta` in between. On the next turn, these parts are loaded from
the DB and sent to Anthropic, which rejects them.

## Fix

Filter empty/whitespace-only text and reasoning parts at the two LLM
dispatch boundaries, without modifying persistence (the raw record is
preserved):

- **`partsToMessageParts()`** in `chatprompt.go` — filters when
converting persisted DB messages to fantasy message parts for LLM calls.
This is the last gateway before the Anthropic provider creates
`TextBlockParam` objects.
- **`toResponseMessages()`** in `chatloop.go` — filters when building
in-flight conversation messages between steps within a single turn.

Note: `flushActiveState()` (the interruption path) already had this
guard — the normal `TextEnd` streaming path did not, but since we're not
changing persistence, the fix is applied at the dispatch layer.

coderd/chatd/chatloop/chatloop_test.go

@@ -578,6 +578,84 @@ func TestToResponseMessages_ProviderExecutedToolResultInAssistantMessage(t *test
 	assert.False(t, localTR.ProviderExecuted)
 }
 
+func TestToResponseMessages_FiltersEmptyTextAndReasoningParts(t *testing.T) {
+	t.Parallel()
+
+	sr := stepResult{
+		content: []fantasy.Content{
+			// Empty text — should be filtered.
+			fantasy.TextContent{Text: ""},
+			// Whitespace-only text — should be filtered.
+			fantasy.TextContent{Text: "   \t\n"},
+			// Empty reasoning — should be filtered.
+			fantasy.ReasoningContent{Text: ""},
+			// Whitespace-only reasoning — should be filtered.
+			fantasy.ReasoningContent{Text: "  \n"},
+			// Non-empty text — should pass through.
+			fantasy.TextContent{Text: "hello world"},
+			// Leading/trailing whitespace with content — kept
+			// with the original value (not trimmed).
+			fantasy.TextContent{Text: "  hello  "},
+			// Non-empty reasoning — should pass through.
+			fantasy.ReasoningContent{Text: "let me think"},
+			// Tool call — should be unaffected by filtering.
+			fantasy.ToolCallContent{
+				ToolCallID: "tc-1",
+				ToolName:   "read_file",
+				Input:      `{"path":"main.go"}`,
+			},
+			// Local tool result — should be unaffected by filtering.
+			fantasy.ToolResultContent{
+				ToolCallID: "tc-1",
+				ToolName:   "read_file",
+				Result:     fantasy.ToolResultOutputContentText{Text: "file contents"},
+			},
+		},
+	}
+
+	msgs := sr.toResponseMessages()
+	require.Len(t, msgs, 2, "expected assistant + tool messages")
+
+	// First message: assistant role with non-empty text, reasoning,
+	// and the tool call. The four empty/whitespace-only parts must
+	// have been dropped.
+	assistantMsg := msgs[0]
+	assert.Equal(t, fantasy.MessageRoleAssistant, assistantMsg.Role)
+	require.Len(t, assistantMsg.Content, 4,
+		"assistant message should have 2x TextPart, ReasoningPart, and ToolCallPart")
+
+	// Part 0: non-empty text.
+	textPart, ok := fantasy.AsMessagePart[fantasy.TextPart](assistantMsg.Content[0])
+	require.True(t, ok, "part 0 should be TextPart")
+	assert.Equal(t, "hello world", textPart.Text)
+
+	// Part 1: padded text — original whitespace preserved.
+	paddedPart, ok := fantasy.AsMessagePart[fantasy.TextPart](assistantMsg.Content[1])
+	require.True(t, ok, "part 1 should be TextPart")
+	assert.Equal(t, "  hello  ", paddedPart.Text)
+
+	// Part 2: non-empty reasoning.
+	reasoningPart, ok := fantasy.AsMessagePart[fantasy.ReasoningPart](assistantMsg.Content[2])
+	require.True(t, ok, "part 2 should be ReasoningPart")
+	assert.Equal(t, "let me think", reasoningPart.Text)
+
+	// Part 3: tool call (unaffected by text/reasoning filtering).
+	toolCallPart, ok := fantasy.AsMessagePart[fantasy.ToolCallPart](assistantMsg.Content[3])
+	require.True(t, ok, "part 3 should be ToolCallPart")
+	assert.Equal(t, "tc-1", toolCallPart.ToolCallID)
+	assert.Equal(t, "read_file", toolCallPart.ToolName)
+
+	// Second message: tool role with the local tool result.
+	toolMsg := msgs[1]
+	assert.Equal(t, fantasy.MessageRoleTool, toolMsg.Role)
+	require.Len(t, toolMsg.Content, 1,
+		"tool message should have only the local ToolResultPart")
+
+	toolResultPart, ok := fantasy.AsMessagePart[fantasy.ToolResultPart](toolMsg.Content[0])
+	require.True(t, ok, "tool part should be ToolResultPart")
+	assert.Equal(t, "tc-1", toolResultPart.ToolCallID)
+}
+
 func hasAnthropicEphemeralCacheControl(message fantasy.Message) bool {
 	if len(message.ProviderOptions) == 0 {
 		return false